1. Field of the Invention
The present invention generally relates to a power transistor device and a manufacturing method thereof, and more particularly, to a power transistor device with a super junction and a manufacturing method thereof.
2. Description of the Prior Art
In a power transistor device, power consumption is directly proportional to on resistance (RDS (on)) between drain and source of the device, and thus the power consumption of the power transistor device can be reduced by decreasing the on resistance. Resistance generated from an epitaxial layer used for withstanding high voltage represents the largest percentage of the on resistance. The resistance of the epitaxial layer can be decreased by increasing the doping concentration of the dopant therein; however, the epitaxial layer is used to withstand high voltages, and the breakdown voltage of the epitaxial layer is reduced when the doping concentration is increased, so that ability to tolerate the high voltage of the power transistor device is reduced.
In order to overcome these drawbacks, a kind of power transistor device having a super junction structure is developed to have both high voltage bearing ability and low on-resistance. In conventional power transistor device, several alternately arranged P-type epitaxial layers and N-type epitaxial layers are disposed on the substrate and several parallel PN junctions, which are vertical to the substrate, are therefore formed. In a conventional method for fabricating a power transistor device, a plurality of deep trenches is etched into an N-type epitaxial layer through an etching process. Then, a P-type epitaxial layer is filled into each deep trench. However, when an aspect-ratio of deep trenches increases, it becomes harder to have P-type epitaxial layers conformally fill into the corresponding deep trenches. As a result, some defects are formed at the interface between the N-type epitaxial layer and the P-type epitaxial layers. Furthermore, voids may inevitably appear within the P-type epitaxial layers. Therefore, a voltage sustaining ability of the super junction structure is reduced.
In light of the above, there is a need to provide an improved super junction power transistor device and a fabrication method thereof, which is not constrained by limits of fabricating deep trenches and can reduce the difficulty on epitaxial growth processes.